1. Field of the Invention
The present invention relates to the technology field of battery monitoring systems (BMS), and more particularly to a battery internal resistance measuring device and a method thereof.
2. Description of the Prior Art
With the high development of modern technologies, batteries are getting more and more important and widely used in electrical or electronic products, such as motorcycles, automobiles, computers, cell phones, and so on. Ordinary batteries have a service life about 2-3 years, but that does not mean all commercial batteries can be used for 2-3 years. Therefore, how to monitor the capacity or lifetime of a battery used in an electrical or electronic product becomes an important issue.
Load testing method is often adopted for measuring the residual capacity of a battery in use. FIG. 1 shows a topology diagram of an internal resistance measuring circuit. From FIG. 1, it is understood that the internal resistance measuring circuit 1′ comprises: a current detecting unit 11′, a voltage detecting unit 12′, a switch SW′, and a measurement resistor Rmes′. In order to measure an equivalent internal resistor Rint′ of the battery 2′, a load resistor RL′ is electrically connected to the battery 2′ for making the battery 2′ output a DC current, such that the current detecting unit 11′ is able to sample a current Imes′ passing the measurement resistor Rmes′. As a result, a dropout voltage Vmes′ across the measurement resistor Rmes' can be calculated or directly measured by the voltage detecting unit 12′. It is worth explaining that, a PWM signal is generated for properly controlling an ON/OFF cycle of the switch SW′.
Continuously referring to FIG. 1, and please simultaneously refer to FIG. 2, where three waveform graphs are provided. In FIG. 2, waveform graph (a) depicts a waveform of the PWM signal, waveform graph (b) represents an ideal waveform of an output voltage signal of the battery 2′, and waveform graph (b) shows a real waveform of the output voltage signal. After comparing the two waveform graphs (b) and (c), it is figured out that voltage-dependent Helmholtz capacitance is produced during the discharge of the battery 2′ and causes the real waveform of the output voltage signal become an incomplete square waveform. It is worth explaining that, a short-circuit voltage (i.e., the battery terminal voltage VE′) would be measured by the voltage detecting unit 12′ during the switch SW′ is at ON state, and the short-circuit voltage is marked in waveform graph (c) by “Vsh′”. On the other hand, when the SW′ is at OFF state, the voltage detecting unit 12′ measures an open-circuit voltage from the battery 2′, which is marked in waveform graph (c) by “Vop′”. Therefore, internal resistor Rint′ of the battery 2′ can be calculated by using following mathematic equations:Rint′=(Vop′−Vsh′)/Imes′=((Vop′−Vsh′)×Rmes′)/Vsh′  (1)Imes′=(Vsh′/Rmes′)  (2)
From above descriptions, it is known that the internal resistance measuring circuit 1′ (i.e., load testing architecture) is the simplest framework to measure the internal resistance of a battery. However, when being applied to measure the battery internal resistance, the internal resistance measuring circuit 1′ shows some practical drawbacks as follows:    (1) Owing to the Helmholtz capacitance effect occurring during the discharge of the battery 2′, it is difficult for back-end analog-to-digital converter to precisely find out corresponding sampling time point so as to pick out the open-circuit voltage Vop′ and the short-circuit voltage Vsh′ from the output voltage signal of the battery 2′. Therefore, it is reasonable that the open-circuit voltage Vop′ and the short-circuit voltage Vsh′ measured in the case of very small internal resistor (mΩ) certainly be incorrect, that would result in an error measurement of battery internal resistance.    (2) In order to pick up correct open-circuit voltage Vop′ and short-circuit Vsh′, it needs to continuously prolong the discharging time of the battery 2′ until the waveform of the output voltage signal tends to be stable, such that the back-end analog-to-digital converter can sense correct open-circuit voltage Vop′ and short-circuit Vsh′. However, continuously prolonging battery's discharging time would result in excessive consumption of the battery 2′.    (3) On the other hand, the internal resistance measuring circuit 1′ (i.e., load testing architecture) can just be used for measuring the internal resistance of a specific battery, but fails to measure difference internal resistances from various types of batteries. For instance, the internal resistance measuring circuit 1′ certainly measure an incorrect internal resistance if making a battery with large capacity discharge by small current. Moreover, if the internal resistance measuring circuit 1′ makes a battery with small capacity discharge by large current, the battery with small capacity would be burned out.
From above descriptions, it is clear that there are no improvement methods, approaches or solutions for effectively measuring difference internal resistances from various types of batteries; in view of that, inventors of the present application have made great efforts to make inventive research thereon and eventually provided a battery internal resistance measuring device and a method thereof.